The most efficient protection against influenza infection is vaccination against the circulating strain and it is important to produce influenza viruses for vaccine production as quickly as possible.
Wild-type influenza virus typically grows very slowly in eggs and cell culture. In order to obtain a faster-growing virus strain for vaccine production it is currently common practice to reassort the circulating influenza strain (referred to herein as the vaccine strain) with a faster-growing high-yield backbone strain. This can be achieved by co-infecting cells in cell culture or the amniotic fluid of embryonated hen eggs with the vaccine strain and the backbone strain. Antibodies specific for the backbone strain's hemagglutinin (HA) and neuraminidase (NA) proteins are then added to block influenza viruses which carry the backbone strain's HA and/or NA protein from replicating. Over several passages of this treatment one can select for fast-growing reassortant influenza viruses which contain the HA and NA segments from the vaccine strain and the other viral segments (i.e. those encoding PB1, PB2, PA, NP, M1, M2, NS1 and NS2) from the backbone strain.
The current approaches have several drawbacks. For example, it typically takes about 35 days from the arrival of a new influenza strain to obtain the final high-growth reassortant, which causes delays in the production of influenza vaccines. Furthermore, the need to passage the viruses several times increases the risk for mutations in the HA antigen to occur which can result in an unwanted change of antigenicity. The use of polyclonal antisera to inhibit the propagation of non-reassorted viruses also increases the risk of introducing adventitious viral agents and other contaminants.
It is an object of the invention to provide further and improved methods for producing reassortant influenza viruses.